Core Data & Bindings, [_NSStateMarker intValue] error

I would appreciate any help you can give me on this - I spent ages
trying to solve it and looked everywhere for anything that might help.

I have an NSTableView which is bound to an NSArrayController, which
manages a set of Core Data entities. I also have Add and Remove
buttons, which are connected to the add: and remove: actions of the
controller - a typical use of Core Data and Bindings.

When I add the first entity by clicking the Add button or when
saveAction: is called (and in other circumstances), I often get this
error: